GROWTH-HORMONE INCREASES IGF-I, COLLAGEN-I AND COLLAGEN-III GENE-EXPRESSION IN DWARF RAT SKELETAL-MUSCLE

The effect of short-term treatment with biosynthetic growth hormone (G H) of male dwarf rats was studied in EDL and soleus muscles. In situ h ybridisation revealed that in the untreated dwarf rat collagen I, coll agen III and insulin-like growth factor-I (IGF-I) mRNA is mainly expre ssed by fibroblasts between the muscle fibre areas. Quantitative image analysis showed that, 8 h after a single GH injection: the level of m RNA for all three genes increased compared to the untreated dwarf anim al. IGF-I mRNA levels were similar in normals and untreated dwarf rats but significantly increased 8 h after a single GH injection in EDL (P < 0.01) and soleus (P < 0.001). In untreated dwarf rats, collagen I a nd III gene expression was significantly less than in normal animals ( P < 0.001). Collagen III gene expression also increased significantly 8 h after a single GH injection, in both muscles (P < 0.01). Collagen I gene expression showed significant increases 8 and 24 h after CH tre atment in EDL(P < 0.01), although the increases seen in soleus did not reach significance. The effects of multiple GH injections (one, two o r four) did not appear to be additive. The results of the time course studies are consistent with an intermediary role for IGF-I in the prod uction of collagen in muscle.